Study On Characteristics Of Saline Intrusion And Purging In Marine Sewage Outfalls

Marine sewage disposal with outfalls is an economical and efficient measure to dispose sewage of city life and industry. Saline intrusion is an important factor that decreases the efficiency of outfalls. This problem has not been settled well yet. In this paper, with the method of physical experiment and three-dimensional numerical simulation, flow characteristics and influence factors in outfall systems during saline intrusion and purging were studied systematically. Some useful conclusions have been gained. The main works of the paper are listed as following:(1) A physical experiment system was established in the laboratory. PIV (Particle image velocimetry) technique was selected to measure the internal flow field of saline intrusion and purging. Based on experimental data, the distribution and development of internal flow field of saline intrusion and purging were analyzed. The mechanisms of saline intrusion and purging were clarified.(2) A three-dimensional numerical model was established. This model was based on the FVM on unstructured grids. The RNG k~ε turbulence model was selected to simulate turbulent characteristics. The species conservation equations were applied to considering the stratification and mixing of sewage and seawater.(3) With this three-dimensional numerical model, flow in the diffuser system with square cross section was simulated. The simulation results were compared with that of the existing two-dimensional numerical model and the physical experimental model. The simulation effect of this model on unsteady, stratification of density and turbulent entrainment, three-dimensional characteristics of internal flow in diffuser systems was evaluated.(4) With the three-dimensional numerical model, the intrusion and purging flow in the outfalls with circular cross section was simulated. Through comparison of the numerical simulated flow field and measured flow field in the experiment, the validity of the three-dimensional numerical model to predict flow characteristics in the outfalls with circular cross section was evaluated. The comparison indicated that the three-dimensional numerical model simulated the unsteady process and stratification and turbulent entrainment in theinterface of the two fluids reasonably to a large degree.(5) The internal flow characteristics of saline intrusion and purging were studied with the three-dimensional numerical model. The relationship between the sewage flow and the flow characteristics and intrusion degree was established. The influence of geometrical structure and operation mode on the saline purging flow was studied. A calculation formula that considers resistance characteristics of unsteady multiphase flow was proposed.